Chlorine bridge bond-enabled binuclear copper complex for electrocatalyzing lithium–sulfur reactions

Engineering atom-scale sites are crucial to the mitigation of polysulfide shuttle, promotion of sulfur redox, and regulation of lithium deposition in lithium–sulfur batteries. Herein, a homonuclear copper dual-atom catalyst with a proximal distance of 3.5 Å is developed for lithium–sulfur batteries, wherein two adjacent copper atoms are linked by a pair of symmetrical chlorine bridge bonds. Benefiting from the proximal copper atoms and their unique coordination, the copper dual-atom catalyst with the increased active interface concentration synchronously guide the evolutions of sulfur and lithium species. Such a delicate design breaks through the activity limitation of mononuclear metal center and represents a catalyst concept for lithium–sulfur battery realm. Therefore, a remarkable areal capacity of 7.8 mA h cm−2 is achieved under the scenario of sulfur content of 60 wt.%, mass loading of 7.7 mg cm−2 and electrolyte dosage of 4.8 μL mg−1.


Comments:
(1) In the main text, the Methods section is suggested to include the facilities and parameters for the electrochemical analysis.It is also necessary to report the lithium-sulfur cell preparation and testing parameters in the main text, especially the sulfur loading and content as well as the amount of electrolyte that highlighted in the abstract.
[Suggestion] Please report the important information in the main text.
(2) In the materials analysis, the UV-vis data is suggested to support with its y axis to know the data analysis.In the FTIR analysis, in addition to the shift of C=N and C=C bonds, the spectra seem to show that all peaks are shifted after coordinating with Cu2+.It is suggested to explain this.
[Suggestion] Please check the data in UV-vis and FTIR analysis.
(3) In the electrochemical analysis, the galvanostatic discharge-charge curves of different cycles at 0.2 C for S/Cu-2 and S/Cu-1 is necessary to add the charge curve to complete the data.The rate performance is suggested to add the discharge/charge efficiency.The "C" and "capacity" reported in plots and main text are suggested to be clarified as discharge capacity.
[Suggestion] Please add the missing data to the electrochemical analysis.(4) The electrochemical analysis and the cell performance are obtained from two different cells.The high-loading sulfur cathode in the cell with low amount of electrolyte seem to suffer a low cycling rate and low electrochemical utilization of sulfur in a short cycle life.The development of lithium-sulfur cells with a high amount of sulfur and low amount of electrolyte is important.It is suggested to provide some discussion on this.It is also suggested to include the recent work in making lean-electrolyte lithium-sulfur cell with a high-loading cathode.
[Suggestion] Please make a discussion and reference discussion on the sulfur loading and electrolyte amount.
(5) Chlorine bridge bond-enabled binuclear copper complex is reported for electrocatalyzing lithiumsulfur battery reaction.Since the electrocatalysis reaction is highlighted in the topic, it is suggested to make a discussion on the electrocatalysis reaction by explaining the reaction intermediate and its active energy toward (or compared to) the lithium-sulfur reaction.
[Suggestion] Please give more information on the electrocatalysis reaction.

Our response:
We thank the reviewer for these positive and helpful comments.Our response follows each comment.Additionally, we agree with the reviewer that the currently obtained experimental and theoretical results don't sufficiently support the expression of "synergistic effect of the Cu dual atom sites".Accordingly, we have revised all the expressions of "synergistic effect" in the manuscript to well support the electrochemical performance of batteries.

Our response:
We are appreciative for this comment.We provide HAADF-STEM images of additional areas of samples (Figure S5a and S5c).We also circled the proximal two Cu atoms in the HAADF-STEM images of samples and calculated the distances between them by using the Digital Micrograph software (Figure S5b and S5d).As seen in Figure S5b, the distances between the proximal two Cu atoms in the HAADF-STEM image of Cu-2 are around 0.35 nm, in accordance with the results of single crystal XRD tests.For the Cu-1 sample, the calculated distances between the neighboring two Cu atoms are obviously fluctuant (Figure S5d).The above results demonstrate that the Clbridge pairs maintain the structural stability of proximal two Cu atom in Cu-2 sample.
"The distances between the proximal two Cu atoms in the HAADF-STEM image of Cu-2 are around 0.35 nm, in accordance with the results of single crystal XRD tests (Figure S5a and S5b).For the Cu-1 sample, the calculated distances between the neighboring two Cu atoms are obviously fluctuant (Figure S5c and S5d).The above results demonstrate that the Cl-bridge pairs maintain the structural stability of proximal two Cu atoms in Cu-2 sample."

Our response:
We thank the reviewer for the professional comment.The operando Raman cell device was purchased from Beijing Scistar Technology Co., Ltd.The components of the operando Raman cell device were illustrated in Figure R1 and the Raman measurements were only taken on the cathode side.Typically, a piece of Al mesh with a pore size of 0.5 mm × 0.1 mm was used as the current.The cathode was obtained by the disconnected coverage of the slurry on the Al mesh.After drying, the cathode was cut into small disks with a diameter of 13.0 mm for the operando Raman cell fabrication.Note that the disconnected material coverage simultaneously provides the cathode and catholyte regions for the operando Raman signal collection.Such an operando Raman cell assembling route has been also reported by our previous investigations (Adv.Sci.2022, 9, 2204027; Nano Energy 2021, 89, 106414).The comparison of Raman signals with regards to different samples can be depicted their ability for suppressing shuttle effect.Generally, the catholyte region more clearly reflect the Raman signals of soluble polysulfides than the cathode region owing to the existence of sulfur, carbon and binder in cathode region.Accordingly, we provided the operando Raman spectra of catholyte regions with respects to Cu-1 and rphenGO, as displayed in Figure S14.
"The cathode was obtained by the disconnected coverage of the slurry on a piece of Al mesh with a pore size of 0.5 mm × 0.1 mm.After drying, the cathode was cut into small disks with a diameter of 13.0 mm for the operando Raman cell fabrication.Note that the disconnected material coverage simultaneously provides the cathode and catholyte regions for the operando Raman signal collection.""Operando Raman spectra of catholyte regions with respects to Cu-1 and rphenGO were also shown in Figure S14.The catholyte region of S/Cu-2 shows the weaker soluble LiPS Raman signals along with the discharge proceeding in contrast to the other two samples.Particularly, the Raman signals of LiPSs in catholyte region of S/Cu-2 almost disappear at the end of discharge.These results further confirm the stronger LiPS anchoring ability of Cu-2. 50,51" "50.Shi, Z., Sun, Z., Cai, J., Fan, Z., Jin, J., Wang

Our response:
We are grateful for the reviewer's instructive comment.Based on the DFT results, the negative Gibbs free energy change (∆G) values present thermodynamically spontaneous exothermic steps and the positive values are the thermodynamically endothermic steps.The rate-limiting step in the entire sulfur reduction process is determined by the biggest positive ∆G value.And such rate determining step is closely related to the bond strength between catalyst and sulfur species.Along this line, the configurations of catalyst and sulfur species have a critical effect on the bond strength, leading to the different biggest positive ∆G values, implying the distinct rate determining step for Cu-1 (Li2S4 to Li2S2) and Cu-2 (Li2S2 to Li2S).Such a phenomenon has been also reported by the previous literatures (Nat.Nanotechnol.2021, 16, 166; ACS Nano 2022, 16, 6414).

In the potentiostatic tests, why is the charge capacity obtained so much higher than the discharge capacity? Can the authors comment on the electrocatalytic ability of the Cu sites in both the discharge and charge direction?
Our response: Thanks for the referee's professional comment.The potentiostatic tests were conducted based on the Li2S nucleation and decomposition procedures.In principle, the calculated Li2S precipitation capacity pertains to the contribution of the Li2S4 (by subtracting the contributions of Li2S8 and Li2S6 in the total capacity) (Angew.Chem.Int.Ed. 2022, 61, 2204327; Adv.Energy Mater. 2020, 10,2002271; ACS Nano 2021, 15, 13436).The Li2S decomposition capacity is attained by calculating the contribution of all the Li2S deposit originating from the conversion of Li2S8, Li2S6 and Li2S4 in the nucleation procedure.Therefore, the Li2S decomposition capacity is much higher than the Li2S precipitation capacity (Adv.Mater.2020, 32, 2000315; ACS Energy Lett. 2020, 5, 3041; Energy Storage Mater.2022, 49, 153).In addition, we have added the comment on the electrocatalytic ability of the Cu sites in both the discharge and charge direction.
"Note that the higher activity for the Li-S redox reactions is harvested by Cu-2 than other samples owing to the identified homonuclear dual Cu centers according to the above potentiostatic test results.The proximal binuclear Cu centers in Cu-2 as well as their optimized coordination environment provide more active interfaces for realizing the appropriate electrokinetic control of the Li2S nucleation and decomposition reactions, manifesting the kinetically promoted discharge and charge processes of the practically operated LSBs."

Our response:
We are grateful for the reviewer's valuable comment.The 1-Cuphen for preparing Cu-1 was synthesized with the mole ratio of copper (II) chloride dihydrate and 1,10-phenanthroline as 1:2.And the 2-Cuphen for preparing Cu-2 was synthesized with the mole ratio of copper (II) chloride dihydrate and 1,10-phenanthroline as 1:1.The amount of Cu centers in Cu-1 is the same as that in Cu-2 owing to the consistent usage of copper (II) chloride dehydrate theoretically.Moreover, we used the ICP-OES to experimentally probe the contents of Cu centers in Cu-1 and Cu-2.The ICP-OES measurement results show that the Cu contents in Cu-1 and Cu-2 are 1.16 and 1.18 wt.%, respectively, substantiating the same contents of Cu centers in the two catalysts.According to our exploration results, 1-Cuphen presents superior solubility in ethanol, resulting in the practical difficulty to precipitate the 1-Cuphen single crystal, which is thus unfavorable for the subsequent single crystal XRD tests.Therefore, the appropriate solvents are selected for 1-Cuphen and 2-Cuphen for successfully precipitating the single crystals.
"Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) were conducted to experimentally probe the content of Cu centers in Cu-1 and Cu-2.According to the measurement results, the Cu contents in Cu-1 and Cu-2 are 1.16 and 1.18 wt.%, respectively, substantiating the same contents of Cu centers in the two catalysts."

Our response:
We thank the reviewer for the constructive comment.We have provided the digital graphs of the coated separator with the catalyst slurry, as displayed in Figure R2.The separator side toward the cathode is fully covered with the Cu-2 and binder.And the separator toward the anode is pure in color and no catalyst penetrates the separator.In addition, we also impaled the separator by using a pin and tested the voltages of the batteries with bare, Cu-2 loaded and impaled separator.It can be seen from Figure R2, the batteries with the bare and Cu-2 loaded separator show the normal voltages of around 2.8 V.And the battery with the impaled Cu-2 loaded separator only displays the voltage of around 2.3 V, implying the occurred phenomenon of short circuit.The above results demonstrate that the covered catalyst cannot penetrate the separator and thus cause the short circuit of battery.Such fact has been also reported by previous investigations (Adv.Mater.2022, 34, 2107638; Adv.Funct.Mater.2022, 32, 2204635; ACS Nano 2021, 15, 13436).The EIS profiles of cycled short-circuit Li//Li cell and Li//Li cell with Cu-2 coated separator in Figure R3.In Figure R3a, the observed abnormal curve shape is attained by the Li//Li symmetric cell owing to the

Our response:
We are grateful for the instructive comment.Sulfur cathodes produce the cracks during the longterm cycling owing to the density difference between sulfur and the discharge product of Li2S.The employment of highly active catalysts can rationalize the conversion reactions between sulfur and Li2S, and the possible reasons can be conducted as follows: (i) the sulfur is restrained in the cathode side and the Li2S deposition on the surface of anode is effectively avoided by the Cu-2, leading to the superior structural integrity of the sulfur cathode (Adv.Mater.2022, 34, 2202256; Nano Lett.2022, 22, 3728); (ii) the homogeneous spatial distribution and small size of Li2S resulting from the catalytic effect of the Cu-2 avoid the structure hollowing out largely and benefit to the stress transferring uniformly in the cathode (Adv.Funct.Mater.2021, 31, 2101285; Adv.Sci.2022, 9, 2204027); (iii) the Cu-2 catalyst is favorable for building a strong SEI film, which is thereby beneficial to dictate a better morphology of the sulfur cathode (Adv.Funct.Mater.2023, 33, 2306321).Therefore, the highly active catalyst can help to maintain the structural integrity of sulfur cathode and thus shows the durable performance, which has been also confirmed by other literatures (Adv.Mater.2016, 28, 9551; Adv.Funct.Mater.2021, 31, 2100793)

Our response:
We are appreciated for the valuable comment.In this manuscript, the slurry cast electrodes were assembled for rate and cycling performance evaluation, and the carbon paper-based electrodes were used for tests of Li2S nucleation/decomposition and CV curves of Li2S6 symmetric cells.The slurry cast electrodes were prepared for performance tests based on the industrial production technologies of batteries.The use of commercial carbon paper aims to accurately evaluate the activity of catalysts, and the detailed reasons involves the following aspects: (i) the carbon paper provides 3D frameworks to spatially support catalyst, pledging the sufficient contact of catalyst and sulfur species; (ii) the carbon paper shows nonpolar surface, benefiting to the accurately identify the chemical affinity of sulfur species by catalyst; (iii) the self-supporting design of carbon paper avoid the use of binder, which is favorable for eliminating the interference.Such approaches of electrode preparation for electrochemical measurements have been extensively applied by the previous investigations (Angew.Chem.Int.Ed. 2021, 60, 24558; Adv.Mater.2021, 33, 2103050; Adv.Mater.2018, 30, 1705219).

Our response:
We thank the reviewer for the constructive comment.We have proofread the manuscript and revised some small grammar errors.In addition, we agree with the reviewer that the present experimental and theoretical results don't sufficiently support the conclusion of "synergistic effect of the Cu dual atom sites".Therefore, we have revised all the expressions of "synergistic effect" in the manuscript.

Our response:
We thank the reviewer for these positive and helpful remarks.Our response follows each comment.

Our response:
We thank reviewer for the professional comment.We have provided the facilities and parameters for the electrochemical analysis.And lithium-sulfur cell preparation and testing parameters in the main text, especially the sulfur loading and content as well as the amount of electrolyte that highlighted in the abstract are also added into the revised manuscript."Li2S nucleation/ decomposition tests 0.2 mol L -1 Li2S8 solution was first attained by mixing sulfur and Li2S at a molar ratio of 7:1 in tetraglyme solvent.CP was punched into small disks with a diameter of 13 mm.Cu-1, Cu-2 and rphenGO powder were dispersed into ethanol and then loaded on CP.Cells were fabricated by employing CP-Cu-1 or CP-Cu-2 as the cathode and lithium foil as the anode, 20 μL Li2S8 solution as the catholyte and 20 μL LiTFSI (1.0 mol L -1 ) solution without Li2S8 as the anolyte.The assembled cells were first galvanostatically discharged to 2.06 V at a current of 0.112 mA and then discharged potentiostatically at 2.05 V until the current was below 10 -5 A. The nucleation capacity of Li2S was calculated by the integral area of the plotted curve based on Faraday's Law.For the Li2S decomposition tests, the cells were fabricated by following the same assemble process as the nucleation test.To ensure the complete transformation of LiPSs into Li2S, the decomposition tests were first performed galvanostatically at a current of 0.112 mA until the potential was low than 1.7 V. Then the cells were potentiostatically charged at 2.35 V until the current was down to 10 -5 A. The whole Li2S nucleation/ decomposition tests were carried out on an Ivium Vertex.One Electrochemical Workstation."

"Assembly of symmetric cells
Sulfur and Li2S at a molar ratio of 5:1 were dissolved in a mixture of 1,2-dimethoxyethane (DME) /1,3-dioxolane (DOL) solution containing 1.0 mol L -1 LiTFSI and 2 wt.%LiNO3 to prepare Li2S6 electrolyte.Symmetric cells were assembled by applying CP-Cu-2 as the working and counter electrodes, and the Li2S6 electrolyte usage was 20 μL.Cyclic voltammetry (CV) tests were conducted between -1 to 1 V at the scan rate of 50 and 0.5 mV s -1 .The CV profiles of the symmetric cells were collected on a Metrohm Autolab G204 Electrochemical Workstation."

"Li plating/stripping tests
Cu//Li cells were assembled by using Li foil as the anode, Cu foil as the cathode and different catalyst coated separators, and 20 μL mixture of DME/DOL solution with 1.0 mol L -1 LiTFSI and 2.0 wt.% LiNO3 as electrolyte.Cu//Li cells were discharged at a current density of 1.0 mA cm -2 and a capacity of 1.0 mA h cm -2 , then striped at 1.0 mA cm -2 on a Neware Battery Measurement System.And the Li//Li cells were assembled with two Li foils as the working electrode, plating and stripping at the needed current."

"Electrochemical evaluations
The slurry was obtained by mixing S/electrocatalyst composite, Super P and LA133 aqueous binder with a mass ratio of 8:1:1.The as-achieved slurry was subsequently coated on a piece of Al foil and followed by vacuum drying at 60 ℃ for 12 h.The cathode was cut into circular disks with a diameter of 13 mm prior to use.The sulfur content and mass loading were 60 wt.%, 1.4-1.6 mg cm -2 , respectively.The content of electrocatalyst can be adjusted into 10 and 20 wt.%.The cointype batteries were assembled with Celgard 2500 PP membrane as the separator, Li foil as the anode and mixture of DME/ DOL solution containing 1.0 mol L -1 LiTFSI and 2 wt.%LiNO3 as the electrolyte.The electrolyte/sulfur ratio was 15.0 and 4.8 μL mg -1 for routine and high-load batteries, respectively.Galvanostatic discharge/charge, rate and cycling performance measurements were carried out on a Neware Battery Measurement System in the voltage window of 2.8-1.7 V. CV, LSV and EIS curves were recorded on a Metrohm Autolab G204 Electrochemical Workstation.""As a result, thanks to the proximity effect, the as-derived S/Cu-2 cathode obtains a remarkable initial capacity of 1140.6 mA h g -1 at 0.2 C with negligible decay.Equally importantly, a remarkable areal capacity of 7.8 mA h cm −2 is achieved under the scenario of sulfur content of 60 wt.%, sulfur mass loading of 7.7 mg cm −2 and electrolyte dosage of 4.8 μL mg -1 ."

Our response:
We are grateful for the reviewer's meaningful comment.The Y axis information for UV-vis data has been provided, as displayed in Figure 2a.The shift of all peaks in complexes has been also discussed in the revised manuscript.The formation of Cu-N bonds in complexes tends to change the configuration, symmetry and electron density distribution of the whole ligand, hence leading to the shift of all the peaks.Such a phenomenon has been also reported by previous literatures (J.

In the electrochemical analysis, the galvanostatic discharge-charge curves of different cycles at 0.2 C for S/Cu-2 and S/Cu-1 is necessary to add the charge curve to complete the data. The rate performance is suggested to add the discharge/charge efficiency. The "C" and "capacity"
reported in plots and main text are suggested to be clarified as discharge capacity.

Our response:
We thank the reviewer for the valuable suggestions.Galvanostatic charge curves with different cycles at 0.2 C for S/Cu-2 and S/Cu-1 have been added in Figure 6d and 6e.The Coulombic efficiencies for the rate performance have been also provided in Figure 6a.Following the review's suggestion, "C" and "capacity" have been clarified as "Discharge capacity"."On the proof of this concept, we introduce chlorine (Cl) bridge bond-enabled atomically dispersed Cu-based complexes as the electrocatalysts in LSBs and investigate the battery performance, involving discharge capacity (C), rate capability as well as cycling stability.""Even with a high sulfur loading of 7.7 mg cm -2 , the S/Cu-2 cathode can obtain an areal capacity (CA) of 7.8 mA h cm -2 , which propels the real implementation of highly efficient and remarkably durable LSBs"

It is suggested to provide some discussion on this. It is also suggested to include the recent work in making lean-electrolyte lithium-sulfur cell with a high-loading cathode.
[Suggestion] Please make a discussion and reference discussion on the sulfur loading and electrolyte amount.

Our response:
We appreciate for the reviewer's professional comment.The high sulfur loading and low electrolyte usage are indeed of utmost importance to practically implementation of LSBs.Along this line, we provide the cycling performance of batteries with the high sulfur loading of 7.7 mg cm -2 and low electrolyte usage of 4.8 μL mg -1 at 0.1 C. The battery cycling performance comparison between this work and other reports based on high sulfur loadings and low electrolyte usage has been also listed in a new Table S3.As seen from Table S3, the battery with adding Cu-2 harvests the favorable areal capacity of 7.8 mA h cm -2 and a capacity retention of 84.6% over 50 cycles at 0.1 C under the conditions of sulfur loading of 7.7 mg cm -2 and electrolyte usage of 4.8 μL mg -1 .Note that the cycling lifespan of the high-loaded cathode need to be further optimized towards commercially viable LSBs throughout unremitting endeavor from the comprehensive design of catalysts, electrolytes, binders, etc.And such explorations of key materials for LSBs are also ongoing in our laboratory."As displayed in Figure 6g and S31, with a sulfur loading of 5.1 and 7.7 mg cm -2 , the S/Cu-2 cathode can respectively achieve the superior initial areal capacities of 5.1 and 7.8 mA h cm -2 , and calculate the values of Ea (Figure S22).The related discussions have been also introduced into our revised manuscript.
"The activation energy (Ea) for each conversion reaction step of sulfur intermediates to further reflect the catalytic activity of Cu-2.Electrochemical impedance spectra (EIS) tests under different temperatures were performed at various voltages where critical sulfur reactions occur (Figure S21).Particularly, Nyquist plots at 2.4 and 2.1 V respectively represent the conversion step of soluble LiPSs, and Li2S4 to Li2S2/Li2S.After fitting the circuits, Arrhenius equation was applied to calculate the values of Ea (Figure S22).As displayed in Figure S22d, the S/Cu-2 cathode obtains the lowest the values of Ea for each voltage among the three samples, further corroborating the high catalytic activity of Cu-2 for the whole sulfur conversion reaction."
Figure R1.Components of the operando Raman cell device.

Figure S14 .
Figure S14.Operando Raman spectra of the catholyte regions with regards to S/rPhenGO, S/Cu-1 and S/Cu-2 during the discharge/charge process at 0.2 C.
occurred short circuit phenomenon (Joule 2022, 6, 273).And the EIS profiles of the Li//Li symmetric cells with Cu-2 based separator in Figure R3b present the typical interfacial charge transfer behavior, confirming no short circuit phenomenon occurring in the cell.

Figure R2 .
Figure R2.Digital graphs of bare PP (a, b), Cu-2/PP (d, e) and impaled Cu-2/PP separator (g, h); voltage tests of the assembled batteries with the corresponding separators.

Figure R3 .
Figure R3.EIS curves of short-circuit Li//Li cell (a) and Li//Li cell with Cu-2 coated separator (b) after working for 100 cycles.7.For the slurry cast electrodes, can the authors suggest why more cracking might be observed Am. Chem.Soc.2006, 128, 16515；J.Am.Chem.Soc.2011, 133, 10081; Dalton Trans.2022, 51, 6314)."Notethat the formation of Cu-N bonds in complexes tends to change the configuration, symmetry and electron density distribution of the whole ligand, hence leading to the shift of all the peaks."